In silicon integrated circuit (IC) fabrication, it is often necessary to isolate devices that are formed in a single substrate from one another. The individual devices or circuit components subsequently are connected to other circuit elements to create a specific circuit configuration. This is true for the individual pixels of a CMOS image sensor.
A CMOS image sensor circuit includes a focal plane array of pixel cells, each one of the cells including either a photogate, photoconductor, or photodiode overlying a charge accumulation region within a substrate for accumulating photo-generated charge. Each pixel cell may include a transistor for transferring charge from the charge accumulation region to a floating diffusion node and a transistor, for resetting the diffusion node to a predetermined charge level prior to charge transference. The pixel cell may also include a source follower transistor for receiving and amplifying charge from the diffusion node and an access transistor for controlling the readout of the cell contents from the source follower transistor.
In a CMOS image sensor, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion node accompanied by charge amplification; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge from the floating diffusion node. Photo charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion node. The charge at the floating diffusion node is typically converted to a pixel output voltage by the source follower output transistor. The photosensitive element of a CMOS image sensor pixel is typically either a depleted p-n junction photodiode or a field induced depletion region beneath a photogate.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256.times.256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524 which describe operation of conventional CMOS imagers, the contents of which are incorporated herein by reference.
CMOS image pixels must be isolated from one another to avoid pixel cross talk. In the case of CMOS image sensors, which are intentionally fabricated to be sensitive to light, it is advantageous to provide both electrical and optical isolation between pixels.
Shallow trench isolation (STI) is one technique, which can be used to isolate pixels from one another. In general, a trench is etched into the substrate and filled with a dielectric to provide a physical and electrical barrier between adjacent pixels. Refilled trench structures, for example, are formed by etching a trench by a dry anisotropic or other etching process and then filling it with a dielectric such as a chemical vapor deposited (CVD) or high density plasma (HDP) silicon oxide or silicon dioxide (SiO2). The filled trench is then planarized by a chemical mechanical planarization (CMP) or etch-back process so that the dielectric remains only in the trench and its top surface remains level with that of the silicon substrate.
To enhance the isolation further, ions can be implanted in the silicon substrate in the area directly beneath the trench. However, a drawback associated with ion implantation beneath the trench, as noted, for example, in S. Nag et al., “Comparative Evaluation of Gap-Fill Dielectrics in Shallow Trench Isolation for Sub-0.25 .mu.m Technologies,” IEEE IEDM, pp. 841-844 (1996), is that ion implantation beneath the trench can result in high current leakage. In particular, when ions are implanted in the substrate close to the edges of the trench, current leakage can occur at the junction between the active device regions and the trench.
In addition to the above-mentioned drawbacks, the dominant crystallographic planes along the trench sidewalls, which have a higher silicon density, create a higher density of trap sites along the trench sidewalls compared to silicon/gate oxide interface of a transistor at the silicon surface. Trap sites on dangling bonds or broken bonds can exist at the gate electrode/oxide interface, in the bulk oxide film, the oxide substrate interface, and/or the trench insulation/active layer interface. The trap sites are normally uncharged but become energetic when electrons and holes become trapped in the trap sites. Highly energetic electrons or holes are called hot carriers. Trapped hot carriers can contribute to the fixed charge of the device and change the threshold voltage and other electrical characteristics of the device. As a result of these trap sites formed along the trench sidewalls, current generation near and along the trench sidewalls can be very high. Generation current from trap sites inside or near the photodiode depletion region contributes to the total dark current. Minimizing dark current in the photodiode is important in CMOS image sensor fabrication. Accordingly, it is desirable to provide an isolation technique that prevents current generation or current leakage.